The present invention relates to a method of controlling the moving speed of an ink ribbon in a shuttle dot line printer.
A shuttle dot line printer has a hammer bank accommodating a plurality of print hammers juxtaposed along a print line. To carry out printing, the hammer bank shuttles back and forth in a direction transverse to a sheet of print paper. During the movement of the hammer bank, the print hammers are selectively fired so as to make dot impressions on the sheet of print paper through an ink ribbon.
FIG. 1 is a plan view showing a conventional shuttle dot line printer which includes a hammer bank 1, a shuttle mechanism 2 for shuttling the hammer bank 1 back and forth in directions C and D perpendicular to an advancing direction of a sheet of print paper 3, and an endless ink ribbon 4. The ink ribbon 4 is stored in an ink ribbon cassette (not shown) in folded state and is withdrawn therefrom to pass through a space between the print paper 3 and the hammer bank 1. The ink ribbon 4 is supported by a pair of ribbon drive rollers 5, a guide roller 6, ribbon guides 7, 8, and a braking portion 9, and is uni-directionally transported in the direction B by the drive rollers 5 rotating in the direction A. To prevent the ink ribbon 4 from being slackened, a braking force is imparted on the outgoing ink ribbon 4 by the braking portion 9.
When the hammer bank 1 moves in the direction D, the moving direction thereof is in coincidence with that of the ink ribbon 4, whereas when the hammer bank 1 moves in the direction C, the moving direction thereof is opposite to that of the ink ribbon 4. When the moving directions of the hammer bank 1 and the ink ribbon 4 are opposite to each other, a force is imparted on the ink ribbon 4 by the impingement of the hammers thereon, tending to pull the ink ribbon 4 toward the moving direction of the hammer bank 1. As a consequence, the portion of the ink ribbon 4 confronting the hammer bank 1 moves at a very slow speed or it may even be instantaneously stopped. Since the ink ribbon 4 is being constantly pulled by the drive roller 5, the ink ribbon 4, at locations between the hammer bank 1 and the drive roller 5, is stretched. The slack created by this stretch is eliminated when the hammer bank 1 changes its moving direction from direction C to D.
More specifically, the slack is eliminated when the printing performed during the opposite directional movement of the hammer bank 1 is terminated and hence the ink ribbon 4 is brought to a condition substantially free from the load imparted by the print hammers. In the course of removing the slack of the ink ribbon 4, the moving speed of the ink ribbon 4 instantaneously increases and is then momentarily equal to the moving speed of the hammer bank 1 moving in the same direction D as the ink ribbon 4. When the relative moving speed of the ink ribbon 4 to the hammer bank 1 is zeroed, the same portion of the ink ribbon 4 is subjected to repetitive impingements by the print hammers, thereby lowering the ink density of the printed characters.
In order to prevent this degradation of the printing quality, it has been proposed to reverse the moving direction of the ink ribbon to agree with the moving direction of the hammer bank 1. However, an ink ribbon reversing mechanism necessary to accomplish this effect is complicated and costly. Further, for high speed dot line printers having a printing capability of 300 to 400 Kanji characters per minute, it is technically difficult to reverse the moving direction of the ink ribbon 4 in synchronism with the reversal of the hammer bank 1 which takes place every 40 to 50 milliseconds.
Another proposal for preventing degradation of print quality is to slow down the moving speed of the ink ribbon 4 when the hammer bank 1 moves in the opposite direction to the ink ribbon 4. The rotational speed the ribbon drive rollers 5 is reduced in synchronism with the change of the moving direction of the hammer bank 1 and this slow down movement of the ink ribbon 4 is continued during the opposite directional movement of the ink ribbon 4. However, such a proposal does not solve the problem. In the high speed dot line printers, the hammer bank 1 starts printing within about 5 milliseconds after it turns around at the ends of the reciprocal path. The stretch of the ink ribbon 4 has not yet been completely restored within such a short period of time. Therefore, the accelerated movement of the ink ribbon 4 still occurs even if the speed of the ink ribbon is changed in synchronism with the change in the moving direction of the hammer bank 1. Rather, the moving speed thereof tends to be further accelerated and the lowering of the print density is not eliminated.
In the foregoing description, it is to be noted that the ink ribbon speed is lowered for the purpose of attaining suppression of the stretch of the ink ribbon 4. The ink ribbon speed needs to be returned to the regular speed for the following reasons. If the ink ribbon 4 is constantly moving at the lower speed, the printed characters tend to become blurred resulting from increased number of print hammer impingements on a unit length ink ribbon 4 in comparison with the case where the ink ribbon 4 is moved at the regular speed. Further, if the ink ribbon 4 is constantly moving at the lower speed, the ink ribbon base cloth would be damaged due to the increased number of impingements of the print hammers thereon. The ink ribbon base cloth may be perforated or frayed, with the result that smooth transportation of the ink ribbon 4 cannot be assured.